Robson was in Bangkok when he had the operation on March 3 and United have said specialists are optimistic about the chances of a full recovery.
Manchester United statement: "The laboratory report revealed a cancerous tumour, which will need further treatment. The oncologist is confident that the condition is treatable.
"At this moment Bryan is fine to continue with his role as Thailand coach and his role as ambassador of Manchester United.
"Bryan and his family have requested that he can continue with his treatment in private and ask that their privacy is respected."
He won 90 caps for his country, 65 as captain, despite a succession of major injuries and was dubbed Captain Marvel by Sir Bobby Robson.
Born in Chester-le-Street, he was a Toon fan from an early age.
I produced a Newcastle Utd fanzine, and each issue we contacted famous Geordie supporters and gave them a dozen questions.
The question "Which club do you hate most?" was always bypassed by the likes of Brenden Foster, Jimmy Nail, Sting, etc ... but Robson came straight out with the answer: "The Mackems, I always have hated them!"
We wish you well Bryan, and I honestly mean that.